The adhesive layer of almost every heat-activatable adhesive tape now on the market has a rather narrow melting point range. Such a tape is called a "hot-melt" adhesive tape, because its adhesive layer is melted at the bonding temperature. At temperatures within 10.degree. C. below the bonding temperature, the adhesive may not even be tacky. Because the adhesive has virtually no strength at the bonding temperature, a clamp may be used until a strong bond develops, either by cooling or by curing. If the bond is later heated to a temperature at or above the bonding temperature, the bond fails unless the adhesive has become cured, e.g., by crosslinking. If the adhesive becomes cured when heated, the tape tends to have a limited shelf life, especially if stored in ordinary warehouses which may become quite hot during the day.
Pressure-sensitive adhesive tapes can be formulated to provide immediate bonding strength, usually called "wet-grab". Bonds tend to retain good strength at moderately elevated temperatures, but may weaken to the point of failure at higher temperatures. One such tape has a heat-shrinkable backing as disclosed in U.S. Pat. No. 3,718,495 (Tomita). After being wound upon an object as a protective covering, heat is applied to cause the backing to shrink tightly around the object. However, its adhesive layer may become so soft upon further heating that the outermost wrap may become loose.
U.S. Pat. No. 3,475,213 (Stow) discloses a pressure-sensitive adhesive tape having an electrically-conductive backing and an adhesive layer containing electrically-conductive metal particles which preferably are flattened. When applied to electrical conductors such as are on one face of a printed circuit board, the particles conduct electricity between the conductors and the backing. Under conditions encountered in use, the particles tend to move about, gradually resulting in increased electrical resistance unless the assembly has a permanent clamp, but this is expensive.
U.S. Pat. No. 4,199,646 (Hori et al.) discloses tapes which are heat activatable and pressure sensitive at room temperature. The adhesives utilized therein are a mixture of acrylate copolymers with monomers containing functional groups and a compatible reactive resin. The heat activatable condition of Hori et al means that heat causes the reactive groups of the acrylate to react with those of the reactive resin, resulting in firming and crosslinking of the adhesive composition, i.e., thermosetting. Tapes according to the present invention do not contain a reactive resin and are substantially non-crosslinkable and substantially non-thermosettable.
U.S. Pat. No. 4,045,517 (Guerin et al.) discloses polyacrylic hot melt adhesives prepared by blending a soft part "A", an acrylate polymer having T.sub.g 's between -85.degree. and 0.degree. C., and a firm part "B", an acrylate polymer having T.sub.g 's between 20.degree. and 150.degree. C. From the disclosure one can conclude that the adhesives are pressure sensitive at ordinary room temperature. Forming a single polymer from the ratios of monomers in Guerin et al affords very poor, if not totally unsuitable, hot melt adhesives. This is contrary to the present invention wherein a single acrylate polymer within the claimed range of T.sub.g can be utilized.
Shear value data presented in Table I herein confirms that tapes according to the present invention are substantially non-crosslinkable and substantially non-thermosettable. This data also illustrates the superior performance of adhesives according to the present invention over those of Hori et al and Guerin et al.